Ive got a winter road bike (Carrera tdf) with Shimamo r500 wheels on. I want to try some cyclocross tyres on them through the winter. The rim width is 15mm, so what width tyre should I be aiming for? Theres plenty of frame/fork/brake caliper clearance. What maje of tyre too, its all new to me this cx style.

For trail riding: http://www.schwalbe.com/gb/offroad-reader/racing-ralph.html, though if its a road bike i doubt that they’d fit.

I quite like the Maxxis Raze, found they worked well on both road and through mud.

As above, can’t imagine they’ll fit, may look like there is a lot of clearance but you’ll be surprised how much bigger cx tyres are.

You don’t want it for CX so why bother with a CX tyre? I doubt you will get one in the frame anyway as they are all about 32mm with lumps on them. As an example my 33mm knobblies won’t fit under the mudguards where a 38mm road tyre fits with room to spare. If its just some off-roading use very puncture proof touring/hybrid tyres. They will be way more puncture proof, last much longer and handle better on almost every surface short of off camber grass, which is what a CX course mostly is.

A cx tyre won’t fit through you brakes and / or frame.

mattsccm, I think you’ve summed up what I want the tyres for.
What size ‘touring’ or ‘hybrid’ tyres could I use on a 15mm rim?

It’s the brake caliper fitting around the tyre that’s more likely to be the limiting factor

What size ‘touring’ or ‘hybrid’ tyres could I use on a 15mm rim?

anything up to 32mm should be okay. It’s not the rim width that is the constraint, it’s the frame/brakes, as said.

Look on the tripster atr thread, several people using 28mm tyres for touring and gravel type stuff.